This invention relates to the processing of cyclically recurring signals to determine the time between successive signal cycles. The invention is useful, for example, in conjunction with a non-invasive arrangement for detecting the periodic pathwise movement of a ferromagnetic element enclosed within a non-magnetic housing of a gas meter.
Gas meters for domestic and industrial use are typically of the positive displacement diaphragm type. Such a meter conventionally includes a housing with a vertical central partition dividing the housing into two sets of measuring chambers. Within each chamber is a flexible bellows (or convoluted sleeve diaphragm) connected to a central crankshaft by means of connecting rods. The crankshaft actuates a valve system which admits gas in and out of the bellows. The bellows is caused to expand and contract by the passage of gas through the meter and act in the same manner as a piston to accurately displace a fixed volume of gas for each stroke, or cycle, of the bellows.
One complete cycle of the bellows produces one turn of the crankshaft. In addition to being connected to the valve system, the crankshaft is also connected, through gearing, to a mechanical counter on the front of the meter. This counter is conventionally known as an index and usually contains one circular sweep hand for testing the accuracy of the meter. The dial including this circular sweep hand is commonly called the proving dial. On typical household domestic type meters, the proving dial indicates a flow of two cubic feet of gas through the meter for each turn of its sweep hand. However, due to the connecting gearing, the crankshaft usually makes eighteen turns for each turn of the proving dial sweep hand. Thus, eighteen complete cycles of the bellows are needed for one cycle of the proving dial sweep hand.
Meter accuracy is determined by measuring the actual volume of gas flowing through the meter for each turn of the proving hand. Meters are usually tested at 20% and 100% of their flow rate capacity. Since a domestic type meter normally has a capacity of 250 cubic feet per hour, it requires two minutes and twenty-four seconds for the proving hand to indicate two cubic feet at the 20% capacity rate. This is extremely time consuming when testing meters on a production basis. It would therefore be desirable to have an arrangement for reducing the amount of time required to test meters for accuracy, in particular by being able to sense internal meter motion so as to detect a single turn of the crankshaft. At the 20% capacity rate, this would take only eight seconds and at the 100% capacity rate, only one and six-tenths seconds. Therefore, by basing the meter accuracy test on only several turns of the crankshaft (i.e., several cycles of the bellows) considerable time could be saved in proving the meter accuracy. It would also be desirable to have an arrangement which is non-invasive in nature so that the motion sensing arrangement does not have any effect on meter operation.
U.S. Pat. No. 4,848,148, the contents of which are hereby incorporated by reference, discloses such an arrangement wherein cycles of operation of a positive displacement diaphragm gas meter are detected by setting up a magnetic field which is directed through the non-magnetic housing of the meter toward an internal cyclically moving ferromagnetic element (e.g., the bellows pan) of the meter. The strength of the magnetic field varies as the ferromagnetic element moves toward and away from the source of the magnetic field, and the magnetic field strength is detected by a Hall effect sensor. An output signal from the sensor, which is representative of the magnetic field strength, is processed to define cycles of operation of the meter. However, the steady state output voltage of the Hall effect sensor is dependent upon the ferromagnetic properties of the ferromagnetic element of the diaphragm meter to which it is in close proximity. Further, the output voltage of the Hall effect sensor changes as the ferromagnetic element moves toward and away from the sensor. This overall change in voltage is dependent upon the ferromagnetic properties of the ferromagnetic element and can vary from as little as 30 mv peak-to-peak to as high as several volts peak-to-peak between meters of different size and type. It is therefore an object of this invention to provide an improved arrangement for processing the cyclic output signal from a Hall effect sensor in an arrangement of the type described in such a way as to be insensitive to noise and signal level so that the same motion sensing arrangement can be utilized with different size and type meters.
Present day state of the art meter testers (or provers) are typically computer controlled. If the computer was required to monitor the testing on a continuous basis, this would be very inefficient because the computer would not be available for other tasks. It is therefore another object of this invention to provide a signal processing arrangement of the type described wherein a computer is used in a supervisory manner to control precision timing hardware which processes the sensor output signal, the computer setting up the conditions under which the timing hardware determines when to start and stop the timing operation.
Meter testing apparatus should preferably be universal--that is, it should accommodate different size meters. However, different size (capacity) meters have different characteristics which affect the output signal of the Hall effect sensor. To a lesser extent, the output signal differs between meters of the same size. Further, the output signal changes as a function of ambient conditions. It is therefore a further object of this invention to provide a signal processing arrangement of the type described which is adaptive so that it operates effectively independent of the meter being tested or the ambient conditions.
Not all diaphragm meters are of the type wherein the position of an internal ferromagnetic element can be monitored. For example, some diaphragm meters have a ferromagnetic housing. However, it has been found that differential pressure measurements across a diaphragm meter have cyclic characteristics caused by valve openings and closings. It is therefore still another object of this invention to provide a signal processing arrangement of the type described which can be utilized generally to determine the time between equivalent points of successive cycles of a cyclically varying signal, independent of the origin of the signal.